The field of this invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to measurement of blood vessel wall compliance by measuring wave speed using an MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0) directed along the z-axis of a Cartesian coordinate system, the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field and consequently precess about the polarizing field in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to an excitation magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation field B.sub.1 is terminated, and may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In addition to producing anatomic images, NMR signals can be produced which indicate the motion of spins. Such motion-encoded NMR signals rely upon the fact that an NMR signal produced by spins moving through a magnetic field gradient experiences a phase shift that is proportional to velocity. For flow that has a roughly constant velocity during the measurement cycle the change in phase of the NMR signal is given as follows: EQU .DELTA..phi.=.gamma.M.sub.1 v
where M.sub.1 is the first moment of a bipolar, motion encoding magnetic field gradient, .gamma. is the gyromagnetic ratio and v is the velocity of the spins. To eliminate errors in this measurement due to phase shifts caused by other sources, it is common practice to perform the measurement at least twice with different magnetic field gradient moments as described in O'Donnell U.S. Pat. No. 4,609,872, issued Sep. 2, 1986 and assigned to the instant assignee. By performing two complete scans with different magnetic field gradient first moments and subtracting the measured phases in the reconstructed image at each location in the acquired data array, a phase map is produced which accurately measures the velocity of moving spins.
Blood vessel wall compliance is an important physiological parameter since it influences the load placed on the heart and is a predictor of coronary heart disease. Furthermore, compliance changes in association with certain risk factors are a predictor for atherosclerosis and may play a role in development of the disease. While aortic wall compliance is an important factor in diseases such as Marfan's syndrome, in general, changes in compliance of the aorta do not correlate well with changes in compliance of the smaller systemic vessels where most blood vessel disease is found.
Several methods have been used in the past to measure wave speed and blood vessel wall compliance, or distensibility, using magnetic resonance. These methods rely on measurement of the time-course of velocity at two different locations along a vessel. Wave speed is determined by measuring the time delay between a selected feature (e.g. the onset of flow) between the two stations. Compliance, or distensibility, D of the vessel can then be computed using the equation: ##EQU1##
where .rho. is the density of blood and C is the measured wave speed.
Unfortunately, all of the methods proposed to measure wave speed to date are either not well suited for measurement of wave speed in smaller blood vessels, or require relatively long acquisition times. Mohiaddin, et al. proposed a method in "Aortic Flow Wave Velocity: The Effect Of Age and Disease", Proceedings of the Seventh Annual Meeting of the Society of Magnetic Resonance Imaging, 7 (suppl 1): 119, 989, which relies upon the aortic arch traversing an imaging plane at two locations. The Mohiaddin et al. method, however, is limited to the aortic arch and is not suitable for measurement in relatively straight blood vessels. A method in which blood velocity is measured along the length of the aorta using m-mode MR imaging is disclosed in Cline et al. U.S. Pat. No. 4,995,394, issued Feb. 26, 1991 and assigned to the instant assignee, but it is not clear how well this method will work for smaller vessels because of geometric considerations. Comb excited Fourier velocity encoding as described in Dumoulin U.S. Pat. No. 5,233,298, issued Aug. 3, 1993 and assigned to the instant assignee, has been demonstrated to work well for the measurement of wave speed in peripheral vessels, although it would be desirable to further minimize examination times.